1. Field of the Invention
This invention relates to a vertical-swing pendulum means suitable for vibration measurement in seismology and aseismic engineering, and more particularly to a vertical-swing pendulum means having a stable and accurate response to long-period components of seismic vibration.
2. Description of the Prior Art
Generally speaking, pendulum means with a long natural period in the order of several seconds to several tens of seconds has been used in seismology as an instrument for accurate measurement of seismic oscillation, and such pendulum means has played an important role in the study of the internal structure of the earth and the seismic source process. To facilitate such studies, various contrivances have been made heretofore in producing comparatively small pendulums having a sufficiently long natural period. In case of vertical-swing pendulums, special consideration is necessary in suspending the weight of the pendulum itself. Typical conventional methods for suspending vertical swing pendulums are the one devised by Sir J. A. Ewing (to be referred to as the Ewing type suspension, hereinafter) and the one devised by Lucian LaCoste (to be referred to as the LaCoste type suspension hereinafter).
In General, the neighorhood of the seismic source of a large-scale earthquake is exposed to strong ground vibration and considerable damage are caused there. When such largescale earthquake occurred, most of conventional pendulums with a long natural period scaled out or fell instable, so that satisfactory measurement could not be obtained by the long-period pendulums. Accordingly, in the aseismic engineering, the so-called acceleration meter type strong-motion seismograph with a very short natural period of shorter than 0.1 second has been developed, and such seismograph is used throughout the world at the present.
Many strong seismic oscillations have a maximum acceleration with period components of about one second or shorter, so that the currently used above acceleration type strong-motion seismograph can measure such short-period components of the seismic oscillations fairly accurately. However, the acceleration type seismograph has a shortcoming in that its accuracy in measuring long-period components of seismic oscillations is low.
On the other hand, there is a pressing need for accurate measurement and recording of long-period components in strong seismic oscillations, because structures with long natural periods of oscillation such as super-high-rise buildings and large-scale oil tanks have increased recently. To meet such need, a number of improvements have been proposed in the strong-motion seismograph. For instance, the inventors have developed an improved "velocity type strong-motion seismograph" and disclosed it in their Japanese Patent Laying-open Publication No. 138,774/78.
The essence of the inventors' improved velocity type strong-motion seismograph was in a structure including two horizontally disposed symmetrical pendulums which are connected to each other by mutually-crossing flexible wires. In the last mentioned structure, forces in directions different from the movable direction of the pendulums are cancelled, so that such structure provides a small size pendulum means having a comparatively long natural period of oscillation and being stable against strong seismic vibration.
It is noted that a horizontal-swing pendulum means having two pendulums disposed one above the other and connected by mutually-crossing flexible wires was once proposed. However, in the vertical arrangement of the two pendulums, the lower-side pendulum or the inverted-side pendulum tends to be turned by its own weight about its axis of swing, and if the axis of swing is made by a leaf spring, the lower-side pendulum becomes instable, while if it is pivotally supported, the accuracy is considerably deteriorated by solid friction at the pivot. Accordingly, such vertical arrangement of two symmetrical pendulums has never been used in seismograph.
In the above-mentioned inventors' velocity type strong-motion seismograph, the horizontal arrangement of the two symmetrical pendulums connected to each other by mutually-crossing flexible wires avoids the difficulty of the lower-side pendulum of the vertical arrangement. In fact, the inventors' "velocity type strong-motion seismograph" has been used in actual field measurement by immersing the horizontally disposed symmetrical pendulums connected by mutually-crossing flexible wires in a highly viscous oil so as to apply a super damping to it, and a number of excellent seismic recordings have been obtained already by it, so that it has contributed to a new frontier in the study of prediction of strong ground motions.
However, as to the vertical component of the above velocity type strong motion seismograph the weights of the horizontally disposed two pendulums are suspended by the so-called Ewing type suspension as shown in FIG. 1. So that the natural period thereof was restricted to be about one second at the longest, and its accuracy was insufficient for the measurement of long-period seismic oscillation. Thus, there is a need for the development of a longer natural period of the pendulum means without sacrificing its stability, so as to expand the use of the pendulum means in the measurement of seismic ground vibrations.